(a) Field of the Invention
The present invention relates to nickel fine powder which comprises nickel and specific amounts of magnesium and/or calcium dispersed therein as well as a method for preparing the nickel fine powder and more specifically to nickel fine powder which is suitable for use, in particular, as a material for the internal electrodes of laminated ceramic condensers and which is excellent in thermal shrinkage characteristics and a method for preparing the nickel fine powder.
(b) Prior Art
A laminated ceramic condenser is one obtained by alternatively putting ceramic dielectrics and internal electrodes on top of each other, attaching them by applying pressure and then firing the resulting laminate to thus unite these layers. There have been developed techniques wherein base metals such as nickel are substituted for noble metals such as platinum and palladium which have conventionally been used as materials for such internal electrodes and such techniques have gradually been advanced.
Up to now, there have been proposed a variety of methods for preparing such nickel powder. Although a dry method such as vapor phase hydrogen-reduction of nickel salt vapor may be listed as a typical example thereof, a wet method in which an aqueous solution containing a nickel salt is treated with a reducing agent under specific conditions to thus precipitate nickel has a variety of advantages including economical efficiency in view of energy cost required for the production of the nickel powder. Typical examples of such wet methods are those disclosed in, for instance, Japanese Un-Examined Patent Publication Nos. Hei 7-207307 and 7-278619. However, the thermal shrinkage characteristics of nickel fine powder prepared by any method differ from those observed for ceramic substrates, by nature. The thermal shrinkage characteristics of nickel fine powder also vary depending on the particle size of a particular product and when the primary particle size is on the order of 0.2 .mu.m, nickel powder undergoes rapid thermal shrinkage at a temperature of not less than about 400.degree. C., while if it is on the order of 0.5 .mu.m, such rapid thermal shrinkage takes place at a temperature of not less than about 500.degree. C.
Ceramic substrates can roughly be divided into the following three groups depending on their thermal shrinkage characteristics, according to OKAZAKI Kiyoshi (see "Ceramic Dielectrics Engineerings", 1978, pp. 135-137, published by Gakuken Sha):
(i) Those which once undergo thermal expansion and are then sintered while proceeding thermal shrinkage, when the temperature rises; PA0 (ii) Those which are sintered while monotonously undergoing thermal shrinkage; PA0 (iii) Those which are free of any monotonous thermal shrinkage, but whose rate of thermal shrinkage stepwisely varies in the course of the heating or the temperature rise. PA0 (i) Use of nickel fine powder whose particle size is on the order of 0.1 to 1 .mu.m, which has high crystallizability and shows a low degree of thermal shrinkage; PA0 (ii) Addition of a sintering retarder.
In any case, however, it has been proved that the thermal shrinkage of the ceramic substrates takes place at a temperature of not less than 700.degree. C. and it has also been known that some of ceramic substrates cause thermal expansion at a temperature of less than 700.degree. C.
An internal electrode for laminated ceramic condensers has in general been prepared by forming a paste of nickel fine powder as a material therefor, printing the resulting paste on a ceramic substrate, putting a plurality of the printed substrates on top of each other, attaching these substrate to one another by applying pressure and heat and then firing the resulting laminate by heating in a reducing atmosphere.
If the nickel fine powder present in the paste undergoes rapid thermal shrinkage at a temperature in the neighborhood of 400 to 500.degree. C. as has been discussed above, however, various defects such as delamination and crack-formation are frequently observed during firing because of difference in the thermal shrinkage characteristics between the ceramic substrate and the nickel fine powder as a material for internal electrodes and this has become a serious problem. To eliminate these defects, the following measures have been devised:
However, these measures have been, by themselves, limited in the level of improvement of the nickel fine powder in the thermal shrinkage characteristics.
As has been described above, the thermal shrinkage characteristics of the nickel fine powder are different from those observed for the ceramic substrates even if the nickel powder is prepared by any known method such as those described above and accordingly, there has been desired for the development of a technique capable of producing nickel fine powder which is improved so that it has thermal shrinkage characteristics almost identical to those of the ceramic substrates.